As of the 0.21 rebase we have Taproot in Elements. There is some folowup work we need to make this usable

• update sighash algorithm (see doc: add document describing Taproot sighashes #933 for design doc)
• add new opcodes to Tapscript (design doc forthcoming)
• update CHECKSIGFROMSTACK to support variable-length messages
• SIGHASH_NOINPUT?
• add Taproot to dynafed headers
• scope out pre-dynafed Taproot in blockheaders; can we softfork it and how much work would this be?
• add Taproot descriptors (related: descriptors tracking issue Descriptors tracking issue #944)
• add Taproot fields to PSET (needs own design doc / coordination with Bitcoin)
• add Simplicity as a Tapleaf (related: simplicity tracking issue Simplicity tracking issue #945)